body{
  margin: 0;padding: 0;
}
.container{
  width: 300px;
  height: 500px;
  border: 1px solid red;
}
.leftmenu{
  width: 100px;
  height: 100%;
  background: linear-gradient(to right,purple,pink);
  position: relative;
  left: -100px;
  transition: left 1s;
}
.arrow{
  background: blue;
  width: 30px;
  position: absolute;
  left: 100px;
}
.leftmenu:hover{
  left: 0;
}
